Matte Surface Mystery

Konrad Sauer called me the other day with a sharpening question that I couldn’t answer. I hate that. Not the call, but the drawn blank with which I responded to his question. The issue is esoteric but that just makes it more interesting (to me, anyway). Konrad sharpens his blades using a Shapton 15,000-grit stone for the final polish. He says he notices an improvement in the wood surface quality and edge retention when he goes the extra mile (or fraction of a micron, if you will.) He recently tried a natural waterstone that he believes to be about 20,000-grit (natural abrasives can be rather vague in this regard). The surfaces left by the two stones look very different and with that, as Holmes said, the game’s afoot.

Here’s Konrad’s mystery: why is the surface left by the natural stone a uniform matte while the Shapton 15K leaves a polished near-mirror? I was stumped. I’ve noticed something similar when I used wet-or-dry sandpaper for honing but never thought much of it. Now I was on the spot. The sharpening “expert” stumped by one of his customers. I couldn’t come up with any decent reason for this at all so I emailed another expert, Harrelson Stanley of GetSharper.com, the Shapton importer in the USA.

Harrelson’s first point is that “the very best natural waterstones top out at about 10,000 to 12,000 grit” so Konrad’s final polish with his natural stone may not be contributing to his goal in this case. But I know that “near-mirror” starts at much coarser grits so this doesn’t answer the matte question. Harrelson next suggested: “the hardness of these particles is about 6 on the abrasive hardness scale; ceramic is 9 and diamonds are 10. The particles in the natural stone are very round … and do not leave sharply defined geometric walls to the grooves they cut. Therefore they do not refract light very well. The very fine ceramic stones leave extremely fine grooves … with very angular walls that refract light well.” Hence the matte surface. He also gave me a contact: Gary Runyon is a geologist with metallurgical training. I sent him my questions, then he and I had a great conversation that propelled the investigation forward.

Gary told me that the abrasive particles in natural waterstones, being softer and rounder than the grit grains in manufactured stones, have trouble abrading the various carbides in tool steel. The iron matrix is not quite as hard as the carbides it contains so the grains wear it away faster, leaving the carbides as slightly proud, rounded domes. The matte surface is essentially a relief map of the microscopic carbides exposed during honing. The harder and sharper aluminum oxide grains in the Shapton stone cut through the carbides as well as the surrounding matrix, leveling the surface to an optical mirror.

So, where to go with all this? Or… so what? Not sure, but I’ve always found interesting the way things work as they approach their limits. The notion of exposing and making visible the very grain structure of steel is intriguing to me and, I hope, to you, too. And I am intrigued when an action as seemingly simple as scratching steel with a (very small) rock yields unexpected results.

I’m sending Konrad a couple of small blades to hone on the two stones and then send back. I’ll have them photographed with an electron microscope and post the results so we can see what’s going on close up. Stay tuned.

12 thoughts on “Matte Surface Mystery”

Hmmm….
I’ve seen similar disparities with some very fine abrasives leaving a matte surface and others leaving a very relective “mirror” finish. I’ve assumed it had to do with microscopic scale surface features but I’m convinced sharp sided grooves reflect produce a mirror finish. Early stealth technology used faceted surfaces to reflect to reduce radar reflectivity compared to rounded surfaces. And all abrasives leave grooves, some grooves are just smaller than others.

I’m enjoying your blog even as I look forward to the arrival of your new book.

The issue of the matte and mirror surfaces produced by natural and manmade waterstones, respectively, is interesting, but moreso if we could understand what each of these effects has on the functioning of the edge that is produced. I have long understood that natural stones produce a more durable edge, at least in very hard Japanese blades, but that idea developed before Shaptons came around.

And here I thought I would never use the knowledge gained in that mineralogy course I took in college. Now when I sharpen I’ll have to think about Moh’s hardness, cleavage planes, and crystalline geometry. Who says sharpening has to be complex? LOL

Sharpening IS complex! But it needn’t be complicated. The whole process, after all, can be summed up as “rubbing steel on a rock”. You don’t need to understand all the complexities to be successful.

As I worked on The Perfect Edge I was continually amazed at the depth of the rabbit hole I’d fallen into. Here at Hock Tools we wrap blades in a single sheet of highly condensed sharpening instructions, yet I managed to fill over 200 pages with (what I sincerely hope) is useful and interesting information, all related to the subject of sharpening. A simple approach to sharpening, that works for you, is fine. But a deeper understanding of the forces at play — and the piece of steel that is so often between the wood and the woodworker — offers greater efficiency at the sharpening station and a positive effect on your approach to cutting wood.

I think Mr. Runyon is dreaming. None of the pictures of tool edges I have seen over the last 30 years have shown “domes” or even lumps. And all of the pictures I have seen of Shapton sharpening show very deep grooves which would be hard to polish out. This suggests that the Shapton stones have very hard and sharp particles, a harsh stone. Further, we are not talking about refraction here. Refraction occurs when light passes through various media. This is reflection.

Mr. Runyon’s excellent reasoning and knowledge has been shown true by subsequent microscope photos that there is an uneven (kind of bumpy) surface produced by natural honing stones—and that so much praised misty finish is another false “myth” as being something which represents the ultimate in sharpness, it can be very good (and more durable) finish for woodworking tools (relatively obtuse bevel angles) but ultimate sharpness (acute bevel angles, thin edges) has always had a mirror finish. I think the verdict natural vs. synthetic stones for ultimate sharpness in most edged tools (especially knives etc) is clearly in favor of man made sharpening and honing stones.

So where does one go to have something photographed under an electron microscope? Probably not the local photo hut.

I also found that going an extra step in sharpening leaves a more durable edge. I used to believe that it was mere overkill, and the extra refinement would be gone with the first pass of the plane over wood, but experience has shown me otherwise. And if Konrad Sauer agrees, I’m sold. I usually use a strop to do the final honing, rather than a superfine stone.

Have you ever seen any evidence of edge-rounding on EM photos after stropping on leather?

I used an SEM at Sonoma State University for the photos in the book. For these samples, I’ve had an offer from a friend who has access to one at work and has offered to take a couple shots. I wish I had one here so I could explore all the permutations (such as your stropping question; good idea, I’ll see if I can work another one or two in.)

Dear Ron. Maybe the following link is a contribution to the dialogue above. http://www.ksky.ne.jp/~sumie99/togi,process.html It is about sharpening Japanese swords. Maybe one of the finest sharpening processes done by hand in the world. All aspects mentioned above one can see and read about here in the SASHIKOMI, the classical polishing style for the sword. The last polishing is done with NUGUI. This is , as far as I understand a Japanese stone powder mixed with oil. After this, the blade is polished. Then (not always) Migaki work, the result is shining. For Migaki, needles are used, horn powder and a waxball. Well all information is on the link and maybe this can give some ‘light’ to the remarks read above. kind regards

Something else may be up. I have used the same set of waterstones for plane iron/chisel back flattening as I use for edge grinding/sharpening. As with an earlier post, starting with 220 grit W/D sandpaper for lapping begins to approach a mirror finish, albeit with 220 grit scratches still visible. Moving to 800 waterstone, 4000, then 6000 (with Nagura) produces haze on the flat lapped surface (yes I know – this polishing is for appearance, not sharpening). However when grinding primary and micro bevel on the same blades I can get a mirror finish. Could it be that some other physical process is ocurring with more surface area? Trapping a pocket of air or slurry between the lapping surface and waterstone, and thus there isn’t any abrasion occurring (why the chisel/iron back “sticks” to the surface of the stones).With diamond stones I have seen the channels that let the slurry “sun out” and there isn’t the same “suction” holding the tool to the sharpening surface.

I’d like to make some comments from experience. While there may be proud carbides in steels sharpened by natural abrasives, and matte finishes in steels sharpened by natural abrasives, I have serious doubts that the two have anything to do with each other. The reason for this, and all one has to do is buy a hard arkansas stone and polish a tool to figure it out, is that the slowest cutting of all oilstones – a worn in polish stone – will not do the same thing. It will instead leave a bright polish on a tool. Whether or not there are carbides present in quantity (for example, comparing a modern tool to a 200 year old chisel) doesn’t make any difference.

There are two things that will make for a dull bevel – scratches from large abrasives, or natural abrasive stones that have loose particles moving around on the surface. A lot of natural stones (like inexpensive japanese waterstones) have no ability to hold their particles in place. If you look at the edges under a microscope, you see a matte sandblasted finish. If you take a stone of similar fineness that does not release grit as it works (like a nakayama stone) and hone an edge, you will find lines in the bevel, just like you will with a synthetic stone.

Certainly there are some stones (like mid-grit shaptons) that have the ability to put a polish on a bevel with still a fairly coarse edge, but there are no stones that I’m aware of that will go toe to toe with the truly fine polishing stones unless they, too, are willing to hold their top layer grit and install a polish. The finest razor finishers display this tendency (eschers and the hard japanese barber hones), and put a bright polish on the bevel of a razor (and hopefully if a sharpener has skill, they will get that quality worked to the edge). If the same razor finishers are too soft to work without releasing grit, they will not make an edge that is comparable to a high grit high polish synthetic stone (and you’ll find out quickly from the razor community that the experienced users will consider them unsuitable).

The proof of the real quality of the edge is no farther away than a carbon steel vintage razor and a shave straight off of the stone with nothing but a couple dozen strokes on a very smooth leather strop. The natural stone that puts a polish on the edge, and the synthetics in the sub-micron category will make for a very easy shave. The natural stones that leave a hazy edge may shave, but the shaving will not be nearly as easy.

Tim zowada has done excellent work taking pictures of the bevel of either a razor or a chisel, all the way to the edge, and shows stones on a slurry (that being loose abrasive on the surface of the stone) and then the same stones without the slurry as the edge is brought up to a bright polish. None of the very fine edges have a sandblasted or matte look, but it is pretty easy to confirm that the escher and thuringian stones on slurry leave an edge that has room for improvement.

This “matte but really sharp” stuff has existed only in the woodworking community where the user has no real ability to detect just how sharp an edge is. In the razor world, it isn’t up for debate because a razor user can figure it out very quickly. Anything that leaves a matte finish needs help either from a modern submicron abrasive strop or a vintage strop that has mild abrasive ability. That will still leave a somewhat dull bevel on a razor, but the edge itself is then the quality of the strop, and not the product of the stone that left the matte finish.

To my knowledge, there is no natural stone with submicron or even micron sized particles, so there is no argument that zowada just needed to use a finer natural stone to get a better edge on slurry (which would result in a matte finish)

That doesn’t mean that a woodworker can’t do great work without ever having a bright polish. Of course that’s not true. But it definitely does mean that for anyone who has the initiative to shave, the reality of what a very fine stone does (impart a bright polish on whatever is sharpened) is very clear.

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The Perfect Edge

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